Air Cylinder
Proximity Switches

Proximity-switches; what are they, and what is their application in pneumatic fluid power? Some applications for air cylinders involve split second timing.

As a result there is a need by the air circuit designer to know exactly where a cylinder rod end is, or of more importance, where the tooling on the rod end is, at all times as the machine cycles.

Machine Timing Can Be Critical

In the event that the machine timing went off, perhaps as the result of a stuck air valve not cycling a cylinder at the right time, or even that the air cylinder itself has failed, then some rod end tooling on the end of one cylinder could impact the tooling on the end of another. The result may be the destruction of both sets of tooling, expensive for the manufacturer both in replacing the components that were damaged at impact, and the even greater loss of revenue from having a machine down when it was not planned for.

This could be avoided if proximity switches were used.

Some air cylinders are built as proximity switch ready by the air cylinder manufacturer, though not all are.

Making an air cylinder switch ready is accomplished by adding a magnet strip around the circumference of the piston, inside the cylinder barrel.

As the cylinder air piston moves from end to end inside the cylinder barrel, so too does the magnetic strip. The magnetic field from the magnet on the piston passes through the wall of the cylinder barrel, and can be sensed by equipment mounted on the outside of the barrel. This equipment is discussed below.

This first graphic depicts a simple reed switch inside a protective housing.

The reed switch is installed on the outside of the cylinder barrel. As the magnet on the piston inside the air cylinder passes beneath it, the two small reeds inside the switch are pulled together by magnetic force. When they touch, a circuit is made, and a signal is sent to initiate an action. As the piston on the magnet moves past the reed switch, the reeds spring apart, and the outgoing signal stops.

Since the distance from the center of the piston to the end of rod tooling is known, the reed switch would be placed on the cylinder barrel in a location that it would be triggered by the passing magnet when the rod end tooling was in the correct location.

Reed Switch Mounting

Some cylinder manufacturers manufacture their air cylinders with mounting points for reed switches on the outside. If the air cylinder is an NFPA type, the reed switch might clamp on the tie rod, one of the four that hold the end caps on a typical NFPA air cylinder.

In Rodless type band cylinders, some cylinder manufacturers offer barrels with machined or extruded slots formed into the side of the cylinder barrel into which the reed switch can be mounted.

Make sure that you discuss all the mounting options available for a proximity switch from your supplier.

Be careful too to ensure that the barrel material of the cylinder is suitable for transmitting a magnetic signal. Stainless steel barrels are not suitable for through-the-cylinder-wall transmission of magnetic force. As a result, cylinder reed or hall effect proximity switches will not work properly on an air cylinder with a stainless steel cylinder barrel.

The outgoing signal from the reed switch will not normally be powerful enough to do work. Quite often the signal from the proximity switch will be sent to a relay in a simple circuit, or to a PLC which can trigger sequential action through relays that can transmit enough electrical power to do work... like energizing a valve solenoid, for example.

Of the proximity switches available to the cylinder user, reed switches are normally the lowest cost.

Reed switches are mechanical, and have a life expectancy measured in the multi-million cycles before failure of the reeds. Reed switches are relatively low cost, simple and effective position sensors.

Hall Effect Switches

In a high cycle application, or the preference for a totally non-contact sensing solution, the Hall Effect type of sensor might be selected.

This drawing depicts a barrel type proximity switch, and the dots represent the "field" generated by the switch.

The Hall Effect switch is often mounted on the rail or slot on the outside of the air cylinder. As the magnet on the cylinder piston passed beneath it, the Hall Effect electrical field is altered, and a signal is generated by that field alteration. When the magnet moved past, the field reverts to its normal state, and the outgoing signal stops.

Hall Effect switches are installed with clamps to attach them to the cylinder tie rods or to fit into slots machined in the cylinder barrel.

Hall Effect type switches have no mechanical moving parts, and as such, will last for many millions of cycles. They are more expensive than reed type switches, but provide longer life in high cycle applications since there is no mechanical movement involved with the switch.

The outgoing signal created when the Hall Effect switch is made is very low strength, and it will need to be amplified in some manner to make it useful.

The barrel proximity switch shown in the drawing is the type that is frequently used to sense the position of the tooling at the end of the rod, rather than a magnet inside the cylinder barrel.

Cylinder Barrel Impervious

What to do when the cylinder barrel does not allow magnetic signals to pass, or the cylinder environment is exceptionally dirty, or the application demands position sensing at the tooling rather than from inside the air cylinder?

There are other devices available, including optical and pneumatic position sensing, to help the air circuit designer in providing position sensing for their difficult air cylinder and tooling applications.

All the various proximity switch or position sensing options have their strengths and weaknesses. There is enough variety that the machine designer should have no problem selecting the sensing option that fulfills these criteria: